1. Background of the Invention
The invention relates generally to the production of high surface area lime or hydrated lime that is directly added to a gas stream to react with gaseous pollutants. More specifically, the invention is directed to a method and apparatus for the on-site production of a high surface area lime or hydrated lime, as well as activated carbon, that are directly added to a flue gas stream produced by a coal-fired boiler or a waste incinerator to remove certain gaseous pollutants such as acid gases and air toxics, such as mercury and other air toxics found in trace amounts.
2. Description of Related Art
Controlling emissions of certain gaseous pollutants, such acid gases and air toxics, in flue gases generated from coal-fired power plants is an important concern. In such flue gases, certain air toxics and other species can be distributed in both the vapor phase and the solid phase, thereby requiring the removal of both solid matter, such as particulate matter, as well as vapor phase species. Further, certain air toxics (e.g., mercury), certain acid gases (e.g., SO3, HCl), and other air toxics (e.g., arsenic, selenium, nickel, lead, organics, etc.) that are present in the vapor phase of such gases are typically found in very low concentrations making removal difficult.
Sorbents are sometimes used to remove certain pollutants from gas streams. For example, sorbents may be injected into a gas stream and collected downstream after adsorbing a targeted vapor phase contaminant. Alternatively, a packed bed, an entrained bed, or fluidized bed may use a sorbent to adsorb a given vapor phase pollutant from a gas stream.
Calcium-based sorbents such as limestone, dolomite, lime, hydrated lime, calcium acetate, and calcium magnesium acetate are used, for example, in the removal of sulfur-based vaporous pollutants, such as SO2 and SO3, from coal-fired power plant flue gases. These sorbents can also be effective in removing other acid gases, such as HCl and H2S, as well as vaporous air toxic species including metal compounds such as selenium, arsenic, nickel, lead, and others found in trace amounts in coal-fired power plant flue gases. However, the process by which hydrated lime or lime, in particular, is produced is energy intensive, making these sorbents relatively expensive.
Regarding the removal of mercury, activated carbon is a sorbent used for sorption of mercury species from coal combustion flue gases and other industrial gas streams. The results from pilot and full-scale field demonstration tests have revealed that at a carbon loading of less than 5 lb/MMft3, mercury removal greater than 90% is achievable for flue gases generated from burning low-sulfur coals. However, mercury removal efficiency for flue gases generated from burning high-sulfur coals, which commonly contain greater than 10 ppm SO3, a level significantly higher than that in flue gases generated from lower sulfur coals, have been reported to be less than 50% at comparable activated carbon loadings. By co-injecting an alkali sorbent such as lime or hydrated lime together with activated carbon, the effectiveness of the activated carbon in a high SO3 gas stream can be significantly enhanced. (See Sjostrom, S., et al.,“Mercury Control with Activated Carbon: Results from Plants with High SO3,” paper # 79, presented at the Mega Symposium, Baltimore, Md., August, 2008, the entirety of which is incorporated by reference herein.)
In view of the foregoing, there is a need for an improved, low-cost method for making lime or hydrated lime to be used as an effective sorbent for removing various gaseous pollutants, including various sulfur oxides, acid gases, and air toxic compounds from flue gases. Further, there is a need for improving the effectiveness of activated carbon as used for removing mercury species from flue gases.